Folic acid is a vitamin required for the synthesis of nucleotide bases and is essential for the proliferation of all cells. Folates also are required for production of S-adenosylmethionine, the common substrate used in methylation of DNA, histones, G proteins, and many metabolic building blocks (see Kim, J Nutr 135:2703-2709 (2005); Loenen, Biochem Soc Trans 34:330-333 (2006)). Almost all cells take in folic acid via the reduced folate carrier or proton coupled folate transporter (see Matherly and Goldman, Vitam Horm 66:403-456 (2003)). Some cells, however, also express a folate receptor (FR) that binds folic acid ˜100,000 times tighter than the aforementioned transporters, and carries bound folates into cells by receptor-mediated endocytosis (see Nakashima-Matsushita et al., Arth Rheum 42:1609-1616 (1999); Turk et al., Arthritis Rheumatoid 46:1947-1955 (2002)). There are four members of the FR family: FRα, FRβ, FRγ and FRδ (see Elnakat and Ratnam, Adv Drug Deliv Rev 56:1067-1084 (2004)). Different isoforms of the FR are used by certain cancer cells, activated macrophages, and the proximal tubule cells of the kidney to capture folates from their environment (see e.g., Nakashima-Matsushita et al. 1999, supra; and Turk et al. 2002, supra). A need exists for reagents and methods for differential targeting of the folate receptors for treatment of disease.